Your plants are infested with a very common greenhouse pest,
Hemispherical Scale, Sassetia coffeae. These scale insects are
parthenogenetic (do not need males to reproduce). Each mature female
lays 600-700 eggs beneath their bodies and then die. The eggs hatch into
crawlers that disperse over the host plant. Once they settle down and
begin to feed, they lose their legs and never move again. They then
develop a thick waxy shell that is impervious to insecticides. They also
produce copious amounts of honeydew- sticky sap like material that
filters down and coats everything below the plant. Then you will often
get black sooty mold growth in the honeydew. While these pests are not
impossible to erradicate, it will take some time and repeated efforts
since the adults are minimally affected by insecticides. My choice of
insecticide would be Orthene, since it is systemic (taken into the plant
tissue). Spray the plant once every two weeks, for at least 2 months.
The other option is to use isopropyl alcohol (rubbing alcohol). Dip a
cotton swab in alcohol and then lightly touch each scale. You will get
even better results if you can loosen or lift the shell of the scale
first. If this is too labor intensive, you can try spraying the alcohol
on the plant, but first try it on a few leaves to make sure the alcohol
will not damage the plant tissue. New leaves will be the most sensitive.
Alcohol can be used on many houseplants safely, such as Boston or
Maidenhair ferns, parlor palms and Oxalis/ Shamrock plants. But it will
damage the young leaves of lemon balm, wax begonia and African violet.
Good Luck!
(Information taken from "The Scale Insects of Pennsylvania Greenhouses"
and Rodale's "Organic Gardening", Feb 1986 issue.)